This invention relates to a method for exciting a surface acoustic wave on a substrate and measuring a property of a target substance placed on a detection part on the substrate on the basis of change in a characteristic of the surface acoustic wave.
As a method for measuring interactions of biological substances such as DNA and proteins, and for measurements using antigen-antibody reactions, surface acoustic wave devices are being used (see for example JP-A-06-133759 and “A Love wave sensor for (bio)chemical sensing in liquids” Sensors and Actuators A, 43(1994)38-43).
In this measurement using a surface acoustic wave device, either the center frequency f0 of an acoustic wave excited in the device (the point at which the conductance of the measuring system is maximal or the point at which the phase is 0) is measured continuously or the center frequency f0 is measured continuously using a network analyser or the like, and the mass load of the surface acoustic wave device is measured on the basis of fluctuation of this center frequency f0.
Now, when a target substance is placed on the detection part of a surface acoustic wave device for measurement, besides the mass load, a viscous load caused by changes in viscosity affects the center frequency f0, and it has not been possible to distinguish these loads clearly. When an attempt is made to investigate interactions of biological materials such as DNA and proteins or to investigate antigen-antibody reactions on the basis of frequency fluctuations with this kind of measurement system, because the viscosity of the target substance introduced into the measurement system and the viscosity of a buffer liquid used (a biochemical buffer liquid having NaCL or KCl or the like as a main constituent) often differ, it has not been possible to distinguish whether the measured frequency fluctuation is a result of mass load caused by bonding of DNA/proteins or bonding of antigen-antibodies or a result of viscous load, and it has not been possible to make precise measurements. And also because of liquid viscosity changes accompanying liquid temperature changes caused by room temperature changes and the introduction of target substances, sometimes precise measurements have not been possible.